The movement of a stairlift carriage along a stairlift rail must be carefully controlled. This is particularly so in the case of curved stairlifts. Not only must the speed of the carriage along the rail be controlled within pre-determined limits, but also care must be taken to ensure that, as the stairlift carriage traverses transition bends in the stairlift rail, the chairlift chair does not depart from the horizontal to a noticeable extent.
By regulation, stairlifts must have some form of device which prevents the stairlift carriage going “over-speed”. This is typically provided in the form of a mechanical over-speed governor mounted within the carriage which, in the event the stairlift carriage exceeds a pre-determined speed, displaces under centrifugal force, engages the rail, and brakes the carriage from further movement.
Whilst these forms of over-speed governor have been in use for many years, they are not without their problems. Because of manufacturing tolerances, they can be unreliable, and trigger at speeds below the intended trip speed. The consequences can be particularly inconvenient as some installations require a serviceman to be called out in order to release and re-set the over-speed governor.
The problem outlined above is exacerbated by the demand for stairlifts to operate at higher speeds. Since the speed at which the over-speed governor should trip is enshrined in regulation, these higher traveling speeds mean that the normal operating mode is very close to the over-speed governor trip speed. This further increases the likelihood of spurious tripping.
An alternative form of over-speed governor to the centrifugal type, is described in UK Patent Application 2,339,419. The governor described in this patent does not operate under the influence of centrifugal force but, rather, senses the carriage speed electronically. When an over-speed situation is sensed, a solenoid is de-energised. This, in turn, releases a braking member which brings the carriage to a halt.
Whilst the electronic speed sensing feature described in UK Patent Application 2,339,419 addresses the triggering unreliability of conventional governors operating under centrifugal force, the particular form of device described in this patent presents other problems. Firstly, the operation of the device described in UK Patent Application 2,339,419 relies upon the brake release solenoid being kept in a static, energized, configuration at all times. When any mechanical or electromechanical component is kept in a set position for long periods of time, it is susceptible to seizure. This is of particular concern in the case of a component forming part of a safety circuit, such as a stairlift over-speed governor. Further, if an occupier vacates the premises in which the stairlift is fitted, say by going on vacation, then a supply of power must be left on to ensure the over-speed governor does not trigger in the occupier's absence. As a matter of convenience, this is important because, in the event the governor is triggered, and for whatever reason, the governor described in UK Patent Application 2,339,419 will require the attendance of a serviceman to in order to be re-set.
A further concern arising from the use of centrifugal based mechanical over-speed governors is that there is no known, practical, means of testing these devices when in place on an assembled stairlift as, particularly in the case of a rack and pinion drive system, there is no way of creating or simulating an over-speed situation.
As stated above, curved stairlifts require the provision of a levelling function to maintain the chair level, whatever the angle of the rail with respect to the horizontal. Traditionally this function has been provided by various forms of mechanical arrangement, which have been regarded as ‘fail-safe’. Now, however, there is an increasing trend amongst stairlift manufacturers to effect levelling using a separate electronically controlled, chair levelling motor.
The advent of electronically controlled chair levelling motors has brought with it, concerns about safety. Rightly or wrongly, there is a concern amongst some in the stairlift industry, that electrical or electronics based systems are, inherently, not as safe as mechanical systems. In the particular case of motorized chair levelling, there is a concern about the possibility of the chair going off-level to a dangerous extent, in the event of main drive failure occurring whilst the carriage is traversing a transition bend in the rail.
Typically the stairlift carriage is slowed when passing over a transition bend. Thus, should a failure occur in the drive system whilst the carriage is traversing a transition bend, it will take a greater length of time for the carriage to build up to a speed sufficient to generate the centrifugal forces necessary to trigger the over-speed governor, than would be the case if the carriage were traversing a straight section of rail.
It will also be appreciated that, when the carriage is traversing a transition bend, the chair levelling motor is in action, under careful electronic control.
The programming of the levelling motor control is established, on the basis of an expected carriage speed, so as to maintain the chair level within carefully defined parameters. Should the carriage drive suddenly fail in this instance, and the carriage speed suddenly increase, it is unlikely that the levelling function could react sufficiently quickly, before the carriage was halted by the over-speed governor, to ensure the chair level remained within acceptable limits. Furthermore, the chair levelling mechanism will have its own ‘over-angle’ sensor which, when the chair moves off level by more than say 5°, will cause power to the chair levelling motor to be cut. If, at this time, the carriage is still in motion through a transition bend, the chair will be taken beyond this 5° limit.
It is an object of this invention to go at least some way in addressing the requirements and concerns expressed above; or which will at least provide a novel and useful choice.